An amorphous oxide film consisted of indium oxide, zinc oxide, and gallium oxide allows visible light to pass through, and has a wide range of electrical properties (i.e., electrical properties ranging from those of a conductor or a semiconductor to those of an insulator). Therefore, such an amorphous oxide film has attracted attention as a transparent conductive film or a semiconductor film (e.g., a semiconductor film used for a thin film transistor (TFT)).
In particular, an In—Ga—Zn oxide semiconductor has attracted attention after a paper relating to an In—Ga—Zn oxide semiconductor was published by Nomura, Hosono, et al. (see Non-patent Document 1).
It has been studied to form such an oxide film using a physical film-forming technique (e.g., sputtering, pulsed laser deposition (PLD), or evaporation) or a chemical film-forming technique (e.g., sol-gel method). In particular, a film-forming technique by sputtering has been studied at a practical level since a film can be uniformly formed over a large area at a relatively low temperature. When forming an oxide thin film by a physical film-forming technique such as sputtering, a target consisted of an oxide sintered body is generally used in order to form the oxide thin film uniformly, stably, and efficiently (i.e., at a high rate).
It is desirable that a target used for sputtering exhibit high conductivity, and rarely cause or show an abnormal discharge and nodules. However, it is difficult to produce an In—Ga—Zn target having such characteristics. This is because the properties and the state of the target change depending on the production conditions and the composition, and a change in conductivity or a change in occurrence probability of nodules and an abnormal discharge occurs.
It has been found that it is very effective to reduce the content of the spinel phase represented by ZnGa2O4 in the target composition in order to obtain an In—Ga—Zn target that rarely causes or shows an abnormal discharge and nodules (see Patent Document 1). In Patent Document 1, the content of the spinel phase is reduced by adjusting the specific surface area of an In2O3 raw material powder to 10 m2/g or less, and grinding the raw material powder so that the difference between the specific surface area before grinding and the specific surface area after grinding is 2.0 m2/g or more.